Pressurized fluid containers are in widespread use for packaging and dispensing a variety of fluid products, including liquids, gases, solids and combinations thereof. Under normal operating conditions, such containers perform entirely satisfactorily. However, in the event that the contents of such containers become over-pressurized, either because of improper use, exposure to heat or for any other reason, then a violent rupture may occur. The art has provided a variety of pressure relief devices for aerosol cans to prevent explosion of the pressurized can. Many of these pressure relief devices are in the bottom of the can, while some are in the sidewall or top.
By way of example only, one category of pressure relief devices provides one or more concave regions in the bottom of the can in combination with coined lines of reduced material thickness. In this regard, it is to be understood that the term “coin” or “coining” denotes a process of moving or displacing metal to achieve a desired indentation profile and includes scoring and/or applied pressure techniques as will be well known to those of skill in the art. In operation, the bottom fractures along the coined lines (or curvatures) in response to an over pressurization of the container contents thereby creating vent openings. A controlled and predictable release of pressure is thus provided. One such system which has been found to be highly effective is disclosed in U.S. Pat. No. 7,222,757 to Ferreira et al., the contents of which are incorporated herein by reference in their entirety.
Typically, in the prior venting systems it is desirable to consistently maintain a prescribed coin depth along the fracture lines to provide a defined venting pressure. A coin depth which is too shallow will result in a high pressure release. On the other hand, a deeper coin depth may produce a prematurely low pressure release, and may prompt the development of micro cracks in the remaining relatively thin web at the base of the coined line. Thus, the manufacturing process must be carefully monitored with particular attention to timely equipment (and tooling) adjustments to compensate for tool wear, and, when appropriate, to replace worn coining tools. Carrying out such monitoring requires frequent product sampling and testing. This requires a relatively high level of skill and may significantly increase manufacturing costs. Thus, while prior manufacturing techniques provide pressure relief devices that are highly effective, such techniques also have relatively significant quality control requirements to provide reproducible results.
An additional variable which must be accounted for in the formation of a controlled pressure vent is that the sheet metal from which the device is formed may vary slightly in starting thickness. If the starting material in the region being coined is thinner than specified, then displacement of a defined amount of material during the coining process will produce an underlying web at the coined region which will be thinner than expected. This may result in a slightly lower rupture strength than anticipated. Likewise, if the starting material in the region being coined is thicker than specified, then displacement of a defined amount of material during the coining process will produce an underlying web at the coined region which will be thicker than expected thereby producing a slightly higher rupture strength than anticipated. Accordingly, the thickness of the metal is monitored closely to ensure reproducible venting characteristics.
In the event that venting does not take place at the prescribed elevated pressure level as intended, the pressure inside the container may increase until there is an eventual failure at the bottom. This failure typically results in the bottom of the container bulging out substantially at the center such that the concave dome of the bottom reverses upon itself and eventually fails by bursting to permit the escape of the pressurized contents.
In the past, it has been difficult to achieve ultimate strength levels in the container bottom that allow the container bottom to withstand pressures significantly above the intended venting pressure in the event that venting does not take place. Thus, there has been a relatively narrow difference between the designed venting pressure and the pressure level causing bottom reversal. It may be desirable for the pressure level causing bottom reversal to be significantly higher than the intended venting pressure to account for variability in coining tools and/or material thickness. At the same time, the intended venting pressure should be high enough to accommodate normal pressure variations without premature venting.
Consequently, there is a continuing need for a pressure relief device having a coined pressure vent that can be mass produced in a manner that will provide reproducible venting at desired pressures with a reduced potential for variability arising from inherent differences in coin depth and/or material thickness during the manufacturing process. There is also a need for a pressure relief device having a coined pressure vent that can be mass produced and which provides ultimate strength levels in the container bottom that allow the container bottom to withstand significant pressures before reversal bursting failure in the event that venting does not take place.